<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        var n=45;
        if(n>9 && n<100){
            alert('两位数');
        }
        else{
            alert('不是');
        }
    </script>
</head>
<body>

</body>
</html>